1. Technical Field
This invention relates to systems for concealing information and, in particular, those systems that render a speech stream unintelligible.
2. Description of the Prior Art
The human auditory system is very adept at distinguishing and comprehending a stream of speech amid background noise. This ability offers tremendous advantages in most instances because it allows for speech to be understood amid noisy environments.
In many instances, though, such as in open plan office spaces, it is highly desirable to mask speech, either to provide privacy to the speaker or to lessen the distraction of those within audible range. In these cases, the human ability to discern speech in the presence of background noise presents special challenges. Simply introducing noise of a stochastic nature, e.g. white or pink noise, is typically unsuccessful, in that the amplitude of the introduced noise must be increased to unacceptable levels before the underlying speech can no longer be understood.
Accordingly, many prior art approaches to masking speech have focused on generating specialized forms of masking noise, in an effort to lower the intensity of noise required to render a stream of speech unintelligible. For example, U.S. Pat. No. 3,985,957 to Torn discloses a “sound masking system” for “masking conversation in an open plan office.” In this approach, “a conventional generator of electrical random noise currents feeds its output through adjustable electric filter means to speaker clusters in a plenum above the office space.” Despite such sophistication, in many instances the level of background noise required to mask conversation effectively remains unacceptably high.
Other approaches have sought to provide masking more discretely by deploying microphones and speakers in more complex physical configurations and controlling them with active noise cancellation algorithms. For example, U.S. Pat. No. 5,315,661 to Gossman describes a system for “controlling sound transmission through (from) a panel using sensors, actuators and an active control system. The method uses active structural acoustic control to control sound transmission through a number of smaller panel cells which are in turn combined to create a larger panel.” It is intended that the invention serve as “a replacement for thick and heavy passive sound isolation material, or anechoic material.” While such systems are in theory effective, they are difficult to implement in practice, and are often prohibitively expensive.
Several techniques for performing obfuscation (often termed scrambling) may also be found in the prior art. U.S. Pat. No. 4,068,094 to Schmid et al. describes “a method of scrambling and unscrambling speech transmissions by first dividing the speech frequencies into two frequency bands and reversing their order by modulating the speech information.”
Adopting a somewhat different approach, U.S. Pat. No. 4,099,027 to Whitten discloses a system operating primarily in the time domain. Specifically, “a speech scrambler for rendering unintelligible a communications signal for transmission over nonsecure communications channels includes a time delay modulator and a coding signal generator in a scrambling portion of the system and a similar time delay modulator and a coding generator for generating an inverse signal in the unscrambling portion of the system.”
These methods are effective in producing an obfuscated stream of speech, that when presented in place of the original stream of speech, is unintelligible. However, they are less effective in rendering a stream of speech unintelligible via superposition of the obfuscated stream of speech. This represents a significant deficiency for application to conversation masking in an office environment, where direct substitution of the obfuscated speech stream for the original speech stream is impractical if not impossible. Furthermore, due to the nature of the scrambling, the obfuscated speech stream does not sound speech-like to the listener. In environments such as open plan offices, the obfuscated stream may therefore prove more distracting than the original speech stream.
U.S. Pat. No. 4,195,202 to McCalmont suggests an improvement on these systems that may in fact produce a less intelligible composite stream, but does not address the need for a speech-like scrambled signal. In fact, a specific effort is made to eliminate one of the key features of human speech. An “encoding apparatus first divides a voice signal to be transmitted into two or more frequency bands. One or more of the frequency bands is frequency inverted, delayed in time relative to the other frequency bands and then recombined with the other frequency bands to produce a composite signal for transmission to a remote receiver. By selecting the magnitude of the delay to approximate the time constants of the cadence, or intersyllabic and phoneme generation rates, of the speech to which the voice signal corresponds, the amplitude fluctuations of the composite signal are substantially lessened and the cadence content of the signal is effectively disguised.”
What is needed is a simple and effective system for masking a stream of speech in environments such as open plan offices, where an obfuscated speech stream cannot be substituted for, but merely added to, an original stream of speech. The method should provide an obfuscated speech stream that is speech-like in nature yet highly unintelligible. Furthermore, combination of the original speech stream and obfuscated speech stream should produce a combined speech stream that is also speech-like yet unintelligible.